Title

Author

Keywords

Wind tunnels, Diffusers

Abstract

The primary object of the investigation reported in this paper was to obtain information that would aid in the design of a more efficient diffuser for FTU's tunnel, and thus increase the run time. Presently FTU's four inch supersonic wind tunnel uses a constant area, normal shock, diffuser to recover the fluid pressure after the test section. Also, FTU's tunnel is of the intermittent blowdown type, which provides only a relatively short test time before the storage pressure decreases to a limiting value at which flow in the test section ceases to be supersonic. The use of a constant area diffuser and normal shock pressure recovery has the disadvantage of always entailing a large loss in stagnation pressure. These losses increase as the test section Mach number increases. Since a diffuser employing a system of oblique shocks should have a better pressure recovery than one with a single normal shock, efforts were made to improve FTU's wind tunnel along these lines. Variable area diffusers whose throats can be closed after flow has been established were of interest in this report because of their higher pressure recovery. The maximum run time of FTU's wind tunnel is limited by the overall operating pressure ratio required to maintain supersonic flow in the test section area. If one can reduce the losses in the tunnel, the operating pressure ratio can be reduced. The reduction in operation pressure can result in an increase in run time. In FTU's tunnel, the majority of losses occurs in the second throat area or the supersonic diffuser. Tunnel run time improvement may be required to conduct heat transfer studies or to conduct force, moment and pressure tests. The results of the one-dimensional analyses of a variable geometry supersonic diffuser are very promising in that they show a longer run time can be obtained for FTU's tunnel. By using a variable geometry diffuser, an intermittent blowdown wind tunnel run time can be increased two to three times that of a constant are diffuser at high Mach numbers. At the design Mach number of 4.0, the theoretical run time can be increased 321 percent over the run time of a constant area diffuser. The references cited made it possible to geometrically design a relatively simple, yet efficient contractible wall (convergent-constant area-divergent) type diffuser. Three flagellates were chosen to form the side walls of the adjustable diffuser. The length of the plates were a compromise between mechanical construction requirements and the need to keep the wall convergent angle relatively small for the Mach number range of FTU's tunnel and to minimize energy losses. The first adjustable diffuser plate has an overall length of 14.5 inches. The angle of convergent for design was chosen to be 7 degrees at the design Mach number of 4.0. The second diffuser plate that forms the constant area passage has an overall length of 12 inches. The third diffuser plate that forms the divergent section has an overall length of 13.5 inches.

Notes

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